openssl 对称加密

OpenSSL 是一个开源的加密库，支持对称加密、非对称加密、哈希算法等多种加密方式。对称加密是指加密和解密使用相同的密钥，常见的对称加密算法有 DES、3DES、AES 等。

在 OpenSSL 中，使用 EVP 接口进行对称加密操作。下面是一个使用 AES-256-CBC 算法进行加密和解密的示例代码：

#include <openssl/evp.h>
#include <string.h>

int aes_encrypt(const unsigned char *plaintext, int plaintext_len,
                const unsigned char *key, const unsigned char *iv,
                unsigned char *ciphertext)
{
    EVP_CIPHER_CTX *ctx;
    int len;
    int ciphertext_len;

    /* Create and initialise the context */
    if(!(ctx = EVP_CIPHER_CTX_new())) return -1;

    /* Initialise the encryption operation. */
    if(1 != EVP_EncryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, iv))
        return -1;

    /* Provide the message to be encrypted, and obtain the encrypted output.
     * EVP_EncryptUpdate can be called multiple times if necessary
     */
    if(1 != EVP_EncryptUpdate(ctx, ciphertext, &len, plaintext, plaintext_len))
        return -1;
    ciphertext_len = len;

    /* Finalise the encryption. */
    if(1 != EVP_EncryptFinal_ex(ctx, ciphertext + len, &len)) return -1;
    ciphertext_len += len;

    /* Clean up */
    EVP_CIPHER_CTX_free(ctx);

    return ciphertext_len;
}

int aes_decrypt(const unsigned char *ciphertext, int ciphertext_len,
                const unsigned char *key, const unsigned char *iv,
                unsigned char *plaintext)
{
    EVP_CIPHER_CTX *ctx;
    int len;
    int plaintext_len;
    int ret;

    /* Create and initialise the context */
    if(!(ctx = EVP_CIPHER_CTX_new())) return -1;

    /* Initialise the decryption operation. */
    if(1 != EVP_DecryptInit_ex(ctx, EVP_aes_256_cbc(), NULL, key, iv))
        return -1;

    /* Provide the message to be decrypted, and obtain the plaintext output.
     * EVP_DecryptUpdate can be called multiple times if necessary
     */
    if(1 != EVP_DecryptUpdate(ctx, plaintext, &len, ciphertext, ciphertext_len))
        return -1;
    plaintext_len = len;

    /* Finalise the decryption. */
    ret = EVP_DecryptFinal_ex(ctx, plaintext + len, &len);
    if(ret > 0) {
        plaintext_len += len;
    } else {
        /* Error */
        EVP_CIPHER_CTX_free(ctx);
        return -1;
    }

    /* Clean up */
    EVP_CIPHER_CTX_free(ctx);

    return plaintext_len;
}

其中，`aes_encrypt` 函数用于加密，`aes_decrypt` 函数用于解密。`plaintext` 是明文，`plaintext_len` 是明文长度，`key` 是密钥，`iv` 是初始化向量，`ciphertext` 是密文，`ciphertext_len` 是密文长度，`plaintext` 是解密后的明文。